As optical transmissions of data communications and other signals are becoming more common, it is desirable to reduce the size of optical transmission, optical amplification and other related devices to a minimum by using "integrated optics". Communication between optical waveguide devices normally is accomplished by means of optical fibers which must be capable of efficiently coupling light between devices. Unfortunately, maximum fiber to waveguide coupling occurs when the separation between the fiber end and the waveguide is zero, and the cylindrical fiber mode is centered on the waveguide. In addition, coupling efficiency is affected by the relative mode sizes between the fiber and the waveguide, and a large area mismatch between the circularly symmetric fiber mode and the normally narrow, rectangular shaped waveguide edge reduces the coupling significantly.
It has been recognized that a lens placed on the end of guided wave devices like optical fibers can increase the light coupling to an adjacent device, a desirable result. The area mismatch problem can be attacked by using the lens to form the light beam emitting from the end of the fiber core to a beam which contracts into a waist smaller than the diameter of the core at a distance from the fiber. On unlensed fibers, the core diameter and the core to cladding index difference determines the waist and the waist exists at the end of the fiber. Since the goal of integrated optics is to miniaturize all optical components, the best solution for positioning the lens is to place it directly on the end of the fiber. Several methods of doing this have been reported in the open literature, such as in Cohen, et al, Applied Optics, Volume 13, page 89, 1974. In addition to increasing the coupling efficiency by decreasing the area mismatch, a lens system can achieve maximum coupling without the requirement of bringing the fiber into near contact with the waveguide. In practice, it is extremely difficult to determine when the fiber is within one micrometer of the waveguide edge and yet not touching. When a fiber end actually makes contact with a waveguide, the fiber is usually damaged, yet to obtain efficient coupling without a lens requires one micrometer or less separation so that the waveguide is not any further than necessary away from the actual beam waist at the end of the fiber. When contact damages the end of the fiber, the emitted light is no longer Gaussian in nature and the coupling becomes very poor. By using a lens on the end of the fiber, a new smaller waist is situated away from the fiber end so that not only is the coupling efficiency maximized but the probability of being able to locate the components for the maximum coupling efficiency without damaging the fiber is increased.
Cohen, et al, mentioned above, has reported a process for placing lenses on single mode fibers by first cleaning the fiber in boiling solvents, then placing photoresist on the fiber and baking it, exposing the photoresist with ultraviolet light traveling through the core of the fiber, developing and rinsing the photoresist and baking the new lens formed of photoresist. To produce a lens by this method one must know the thickness of the photoresist prior to exposure so that the amount of light reaching the fiber end and exposure time can be balanced to fabricate the desired lens. If these parameters are not properly controlled, the whole process can fail in at least two ways. First, if there is insufficient light or the exposure time is too short, the photoresist is underexposed and simply washed away during the developing and rinsing process. If the photoresist is overexposured, a "flat top" lens is produced. This is due to the complete exposure of the photoresist over an area centered around the peak of the Gaussian light distribution out the end of the fiber. Therefore, a new process which is simpler and eliminates the need to clean the fiber as well as the need to know the thickness of the photoresist, the precise power of light carried by the fiber, and the exposure time of the photoresist, has been required.